Two unrelated people in the same area suggests a bad lot number. This can mean all sorts of things, that it was not properly refrigerated all the time or otherwise improperly handled, would be the first guess. Second guess is that there was contamination in the syringes.
But an event like this really activates both the US Public Health Service and the FDA. They will not dilly-dally in tracing the source of the problem.
I never heard of a "bad lot" of vitamin D-3. It is much more effective than any flu shots, like 100%, at least among the couple dozen people I have got started on it. This year I gave bottles of it to some of the widows at church. I decided to do that when I heard an 80 year old lady complaining about the shot making her really sore and then getting flu anyway.